1. Field of the Invention
The present invention relates to a method of performing beam compression on an antenna pattern of a radar, which can provide better performance in the beam compression without introducing distortion.
2Description of the Related Art
In general, a beam width is one of indexes representing the performance of an antenna pattern of a receiving antenna or other type antennas. A narrower beam width of an antenna pattern will give a better performance. However, there is a relationship of inverse proportion between a beam width and the size (length) of an antenna. Therefore, if the beam width is reduced, then the size of the antenna will be increased. Conversely, if the dimension of the antenna is reduced, then the beam width will be broadened.
For example, in an antenna for a radar system, if it is desired to double the ability or the resolution to discriminate objects, it is required to halve the beam width and thus it is required to double the size of the antenna. The doubling of the size leads to not only a larger occupied region but also various disadvantages such as an increase in the weight of the antenna and in the size of a structure for supporting the antenna. Conversely, if the size of an antenna is halved, then the beam width will be doubled and the discrimination ability will be degraded by a factor of two.
It is well known that there is such a conflicting relationship between a beam width and the size of an antenna. In most cases, an actual antenna has a limitation in the region it can occupy. Therefore, under these limited conditions, a certain degree of compromise associated with the beam width has to be made.
One known beam compression technique to alleviate the above-described problems is to reduce the beam width by means of multiplication of received signals of a plurality of antennas according to the multiplicative array principle. FIG. 1 is a schematic diagram illustrating a configuration of a radar system which can perform the beam compression in such a manner described above. In this figure, reference numeral 101 designates a main antennas such as an array antenna comprising a plurality of radiation elements which are equally spaced along a straight line. Reference numeral 102 designates a sub-antenna disposed apart from the main antenna 101 in the X-direction which is the direction of a beam-width to be reduced. Reference numeral 103 designates a transmitting circuit which generates transmission power, which is in turn fed to the main antenna 101. Reference numeral 104 designates a multiplying circuit (multiplier) which performs multiplication between a received electric field signal received by the main antenna 101 and a received electric field signal received by the sub-antenna 102. Reference numeral 105 designates a rectifying circuit (rectifier) which provides an output signal only when the multiplied output signal from the multiplying circuit 104 is positive. In FIG. 1, the beam-axis direction is perpendicular to the X-Y plane or the plane of the drawing.
In the antenna system having such a configuration described above, electric field signals received by the respective antennas 101 and 102 are fed in the same phase to the multiplying circuit 104 so as to perform the multiplication on these signals. The multiplied signal is output via the rectifying circuit 105 only if the multiplication result is positive. As a result, a received electric field signal (electric field pattern) corresponding to the directional characteristic of the main antenna 101 such as that shown in FIG. 2 is multiplied by a received electric field signal (electric field pattern) corresponding to the directional characteristic of the sub-antenna 102 such as that shown in FIG. 3, thus providing an output signal (output pattern), such as that shown in FIG. 4, corresponding to the synthetic directional characteristic having a beam width .theta. wc which is reduced from the beam width .theta. w of the electric field pattern associated with the main antenna.
In the beam compression method of an antenna pattern according to the conventional technique based on the above-described multiplicative array principle, there is a problem that it is impossible, as a matter of course, to achieve a resolution better than that corresponding to the beam width .theta. wc of synthetic directional characteristic, shown in FIG. 4, obtained by performing the multiplication process on the received electric field signals.
In the beam compression method according to the multiplicative array principle, since a sub-antenna is located off the scanning rotation axis, a large grating lobe occurs. Similarly, in the case where a pair of sub-antennas are used, a large distance between sub-antennas causes a large grating lobe. As a result, even at angles at which no scattering object exists, if the grating lobe is directed to a scattering object, the sub-antenna produces a rather great received signal due to a received radio wave corresponding to the grating lobe. Therefore, if the beam compression is performed by simply multiplying the received signal associated with the main antenna by the received signal associated with the sub-antenna, distortion occurs in the received pattern.
To solve the above problem, the inventor of the present invention has proposed a beam compression method in U.S. patent application Ser. No 08/217,064 (Japanese Patent Application No. 5-131035). In this previously proposed invention, the beam width compression is performed by multiplying the received power signal associated with the main antenna by that associated with the sub-antenna instead of multiplying the received electric field signal associated with the main antenna by that associated with the sub-antenna thereby achieving better resolution, wherein an output signal is provided only if the cosine value corresponding to the phase difference between the received power signal associated with the main antenna and that associated with the sub-antenna is positive thereby preventing the distortion in the output signal.
In general, the power signal received by an antenna can be represented by the square of the received antenna electric field signal. If the received power pattern is compared to the received electric field pattern, the received power pattern has a less spread pattern, that is, the received power pattern has a narrower beam width than the received electric field pattern. Therefore, in the beam compression method of an antenna pattern according to the multiplicative array principle, if the received power signal associated with an antenna is used instead of the received electric field signal, it is possible to compress the beam to a narrower width. FIGS. 5-7 illustrate a power pattern of a main antenna, a power pattern of a sub-antenna, and an output signal pattern obtained by performing beam compression based on a power signal, respectively.
The phase difference between the grating lobe of the sub-antenna and the main beam of the main antenna, or the phase difference between the main beam and the grating lobe of the sub-antenna is about 180.degree. which is greater than 90.degree.. Therefore, in the case where there is no scattering object in the main beam direction of the main antenna, and there is a scattering object in the grating lobe of the sub-antenna, the phase difference between the received signal associated with the main antenna and that associated with the sub-antenna becomes greater than 90.degree., and thus the signal corresponding to the cosine of the phase difference between the received signal associated with the main antenna and that associated with the sub-antenna becomes negative. Thus, in the beam compression method according to the previous invention, the distortion is prevented by outputting a zero signal in the case where the phase difference between the received signal associated with the main antenna and that associated with the sub-antenna is greater than 90.degree..
However, if the distance between scattering objects is small, the phase difference between the signal received by the main antenna and the signal received by the sub-antenna arising from its grating lobe can be smaller than 90.degree. near angles at which the short-distance scattering objects exist. This means that in the beam compression method which provides an output signal only when the cosine value corresponding to the phase difference is positive, scattering objects located at a short distance from each other cannot be distinguished.